1. Field of the Invention
The present invention relates to gas bearings, commonly known as air bearings, for use in high precision optical and mechanical equipment.
2. Description of the Prior Art
Gas bearings have long been used for translational movement. Such bearings are used, for example, in such high precision equipment as step and repeat cameras for making integrated circuit patterns and other optomechanical devices. U.S. Pat. No. 3,722,996 to Wayne L. Fox for an "Optical Pattern Generator or Repeating Projector or the Like" granted Mar. 27, 1973 shows prior art gas bearings used in a high precision optomechanical device.
An advantageous form for a gas bearing for translational movement comprises a body with a gas pocket in the form of a portion of a sphere in one surface of the bearing. A gas, usually air, is supplied to the pocket at a constant pressure through a passage in the body. The load on the bearing is supported above a smooth fixed bearing surface by the film of gas between the pocket and the bearing surface. Since there is no actual contact between the pocket and the bearing surface and the viscosity of most gases, including air, is quite low, gas bearings are almost frictionless. Such a gas bearing is shown and described in United Kingdom Pat. No. 1,436,254 for "Air Bearing Arrangements."
The extremely low friction of a gas bearing permits a movable element in a piece of equipment to be moved with great precision using very small forces. However, under some load conditions, the prior art gas bearing described above permits undesirable oscillation of the load. This oscillation of the load has been found to occur most commonly when the load is relatively tall, as compared with its width and depth, and includes a mass in the form of a tower-like structure that is relatively compliant. Mechanical shocks can cause such a structure to oscillate at a resonant frequency in the range of 40 and 50 Hz, and the prior art gas bearing does not provide sufficient damping for such oscillations. The prior art mass-bearing system acts like a feedback loop with insufficient phase margin at gain crossover.